Bring steaks to room temperature (20–30 minutes): Remove the tenderloin medallions from the fridge and pat them dry with paper towels. Season both sides with 1 1/2 teaspoons kosher salt and 1 teaspoon freshly ground black pepper. Letting the meat come to room temperature ensures more even cooking and a better crust when searing.

Boil the potatoes: Place the baby potatoes in a pot, cover with cold water by 1 inch, add 1/2 teaspoon coarse sea salt and 2 rosemary sprigs, and bring to a simmer. Boil until tender when pierced with a knife, about 15–18 minutes depending on size. Starting in cold water and including salt seasons the potatoes evenly and helps them cook through without falling apart.

Preheat oven and prepare sheet: Preheat the oven to 425°F (220°C). Drain the potatoes and let them steam-dry in the colander for 2–3 minutes. Place them on a rimmed baking sheet lined with parchment or lightly oiled; using parchment makes cleanup easier and prevents sticking.

Smash and crisp the potatoes: Using a sturdy glass or a potato masher, gently press down each potato until it flattens to ~1/2 inch thickness (don’t pulverize—leave it in one piece). Drizzle or brush with 1 tablespoon olive oil, sprinkle 1/4 teaspoon black pepper and roast in the preheated oven until edges are golden and crisp, 20–25 minutes. Smashing increases surface area for maximum crispness while keeping a creamy interior.

Sear the steaks: Heat a heavy 12-inch skillet (cast iron preferred) over high heat until very hot, about 3–4 minutes. Add 1 tablespoon olive oil and swirl to coat. Place the steaks in the pan without crowding—work in two batches if necessary. Sear undisturbed for 2–3 minutes until a deep brown crust forms, then flip and sear the other side 2 minutes. For medium-rare on 6-oz tenderloin medallions, finish by adding 2 tablespoons butter and spooning the melted butter over the steaks while they finish about 1 minute; the internal temperature should read 125–128°F (52–53°C) for medium-rare. High heat creates the Maillard reaction (browning), which produces intense savory flavor and a satisfying crust; basting with butter adds richness and helps the exterior finish cooking gently.

Rest the steaks: Transfer seared steaks to a warm plate and tent loosely with foil for 6–8 minutes. Resting lets the muscle fibers relax and the juices redistribute—cutting too soon makes them runny and less flavorful.

Make the red wine–balsamic reduction: Wipe the skillet of excess fat, keeping any browned bits (fond) on the bottom—these are flavor gold. Over medium heat, add the remaining 2 tablespoons butter and 2 crushed garlic cloves and sauté briefly (20–30 seconds) to soften and fragrant—don't brown the garlic or it will turn bitter. Pour in 1/2 cup dry red wine and scrape the fond with a wooden spoon to dissolve those caramelized bits into the sauce. Add 1/2 cup beef stock and 2 tablespoons balsamic vinegar. Simmer gently, reducing until the sauce thickens slightly and coats the back of a spoon, about 6–8 minutes. Reduction concentrates flavors; the acid from wine and balsamic balances the buttery beef for a glossy, harmonious sauce.

Finish the sauce and aromatics: Turn off the heat and swirl in the remaining 2 tablespoons butter (cold) and 1 tablespoon chopped thyme to emulsify the sauce and give it a silky sheen. Taste and season with a pinch of salt and a few grinds of black pepper. Cold butter added off the heat smooths and slightly thickens the sauce without breaking it.

Finish the potatoes: Remove the potatoes from the oven when crisp and sprinkle with a little flaky sea salt. If you like, toss with chopped parsley for freshness. Crispy edges with tender centers make an excellent textural contrast to the tender beef.

Plate and serve: Place two smashed potatoes on each plate, set a rested tenderloin medallion beside them, and spoon the red wine–balsamic reduction generously over the steak. Top each medallion with a small pat (about 1 tablespoon) of the garlic-herb butter (you can dollop from the pan or pre-chill and slice). Sprinkle with remaining chopped parsley and a few thyme leaves for color. Serve immediately to enjoy the contrast of hot potatoes, warm sauce, and cool, melting herb butter.